OnlineGlomLoginServlet.checkAuthentication(): Require HTTPS.

OnlineGlomLoginServlet.checkAuthentication(): Require HTTPS.

        * pom.xml: Use HTTPS when running jetty for testing via mvn gwt:run.
	This annoys us with a security warning about the self-signed certificate,
	but it's better than the risk of deactivating the checks.
        * src/main/java/org/glom/web/server/OnlineGlomLoginServlet.java:
        checkAuthentication(): Refuse to check if the servlet is not running via
	HTTPS.
        * src/main/webapp/WEB-INF/web.xml: Add a comment about why we do
	not require HTTPS for the login servlet in all cases.